In recent years, offshore exploration for and production of petroleum products has been extended into arctic and other ice-infested waters in such locations as northern Alaska and Canada. These waters are generally covered with vast areas of sheet ice 9 months or more out of the year. Sheet ice may reach a thickness of 5 to 10 feet or more, and may have a compressive or crushing strength in the range of about 200 to 1000 pounds per square inch. Although appearing stationary, ice sheets actually move laterally with wind and water currents and thus can impose very high forces on any stationary structure in their paths.
A still more severe problem encountered in arctic waters is the presence of larger masses of ice such as pressure ridges, rafted ice or floebergs. Pressure ridges are formed when two separate sheets of ice move toward each other and collide. Pressure ridges can be very large, with lengths of hundreds of feet, widths of more than a hundred feet, and a thickness of up to 50 feet. Consequently, pressure ridges can exert a proportionally greater force on an offshore structure than ordinary sheet ice. Thus, the possibility of pressure ridges causing extensive damage to an offshore structure or the catastrophic failure of a structure is very great.
It has been proposed heretofore that rather than build a structure strong enough to withstand the total crushing force of the ice, that is, strong enough to permit the ice to be crushed against the structure, the structure be built with a ramp-like surface. As the ice comes into contact with such a surface, it is forced upwardly above its normal position, causing the ice to fail in flexure by placing a tensile stress in the ice. Since the ice has a flexural strength of about 85 pounds per square inch, a correspondingly smaller force is placed on the structure as the ice impinging thereon fails in flexure rather than in compression.
Several forms of structures having a sloping peripheral wall are illustrated in a paper by J. V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice Floes," presented to the First International Conference on Port and Ocean Engineering under Arctic Conditions held at the Technical University of Norway, Trondheim, Norway, during August 13 to 30, 1971. Another publication of interest in this respect is a paper by Ben C. Gerwick, Jr. and Ronald R. Lloyd entitled "Design and Construction Procedures for Proposed Arctic Offshore Structures," presented at the Offshore Technology Conference meeting at Houston, Texas during April 1970.
In the far northern artic waters, such as the waters off the north slope of Alaska, the open water season is relatively short, approximately six weeks. After the end of the season, ice begins to form on the open waters where it will freeze around and onto any structure established in the water. This condition has been duplicated in the laboratory to determine what effect the new sheet ice would have on a scale model of a structure having a ramp-like surface and particularly to determine what forces would be imposed on such a structure.
As the ice sheet built up in thickness on the surface of the water surrounding the model structure, it also froze onto that part of the structure's outer surface in contact with the water. When the ice sheet reached the required thickness for the test, it was found that a much greater force was required to start relative motion between the model and the adhering ice sheet than was required to maintain the relative motion after the adhesive bond between the ice and the structure was broken. For the conditions of the test, approximately 5 to 10 times as much force, depending on specific conditions, was imposed on the model structure by the ice sheet before the bond was broken than was imposed after the relative motion was begun.
The amount of the ice force imposed on the structure will, of course, be dependent on the form, dimensions and characteristics of the structure and the dimensions and characteristics of the ice. But in all cases, as the problem is understood now, a much greater force will be imposed on the structure before the adhesive bond between the structure's surface and the ice is broken than will be imposed after the bond is disrupted. That is to say, for the ramp-like surface design to be an effective means for reducing ice forces, the ice must be free to move relative to the structure. Otherwise, it might be expected that the structure would have to be built strong enough to withstand the initial forces imposed thereon as the bond between the ice and the surface of the structure is broken.
It has been found, however, that if the ice is prevented from freezing on and adhering to the structure's ramp-like surface, the structure does not need to be built strong enough to withstand the loads associated with ice-bonding. Accordingly, it has been proposed heretofore that outer surface of the structure be heated to a temperature above the melting point of the ice, or that the outer surface of the structure be made of a material having low ice-adhesion properties. Particularly, U.S. Pat. No. 3,831,385, assigned to the assignee of the present invention, discloses heat exchanger apparatus that uses exhaust gases from engines onboard the structure for heating the sloping surface of the structure to the desired temperature. This patent also discloses that electrical resistance heating may be used to maintain the temperature of the structure's exterior surface above the melting point of the ice. And U.S. Pat. No. 3,972,199, also assigned to the assignee of the present invention, discloses coating or forming the structure's sloping surface of a material that has an adhesion between ice and the structure's surface of between 0 and 100 psi.
The present invention is directed to a different way for heating the exterior surface of a production structure to a temperature above the melting point of ice.